Generally a hearing aid system according to the invention is understood as meaning any device which provides an output signal that can be perceived as an acoustic signal by a user or contributes to providing such an output signal, and which has means which are customized to compensate for an individual hearing loss of the user or contribute to compensating for the hearing loss of the user. They are, in particular, hearing aids which can be worn on the body or by the ear, in particular on or in the ear, and which can be fully or partially implanted. However, those devices whose main aim is not to compensate for a hearing loss but which have, however, measures for compensating for an individual hearing loss are also concomitantly included, for example consumer electronic devices including mobile phones, televisions, hi-fi systems, MP3 players and mobile health care devices comprising an electrical-acoustical output transducer which may also be denoted hearables or wearables.
Within the present context a traditional hearing aid can be understood as a small, battery-powered, microelectronic device designed to be worn behind or in the human ear by a hearing-impaired user. Prior to use, the hearing aid is adjusted by a hearing aid fitter according to a prescription. The prescription is based on a hearing test, resulting in a so-called audiogram, of the performance of the hearing-impaired user's unaided hearing. The prescription is developed to reach a setting where the hearing aid will alleviate a hearing loss by amplifying sound at frequencies in those parts of the audible frequency range where the user suffers a hearing deficit. A hearing aid comprises one or more microphones, a battery, a microelectronic circuit comprising a signal processor, and an acoustic output transducer. The signal processor is preferably a digital signal processor. The hearing aid is enclosed in a casing suitable for fitting behind or in a human ear.
Within the present context a hearing aid system may comprise a single hearing aid (a so called monaural hearing aid system) or comprise two hearing aids, one for each ear of the hearing aid user (a so called binaural hearing aid system). Furthermore the hearing aid system may comprise an external computing device, such as a smart phone having software applications adapted to interact with other devices of the hearing aid system. Thus within the present context the term “hearing aid system device” may denote a hearing aid or an external computing device.
The mechanical design of hearing aids has developed into a number of general categories. As the name suggests, Behind-The-Ear (BTE) hearing aids are worn behind the ear. To be more precise, an electronics unit comprising a housing containing the major electronics parts thereof is worn behind the ear. An earpiece for emitting sound to the hearing aid user is worn in the ear, e.g. in the concha or the ear canal. In a traditional BTE hearing aid, a sound tube is used to convey sound from the output transducer, which in hearing aid terminology is normally referred to as the receiver, located in the housing of the electronics unit and to the ear canal. In some modern types of hearing aids a conducting member comprising electrical conductors conveys an electric signal from the housing and to a receiver placed in the earpiece in the ear. Such hearing aids are commonly referred to as Receiver-In-The-Ear (RITE) hearing aids. In a specific type of RITE hearing aids the receiver is placed inside the ear canal. This category is sometimes referred to as Receiver-In-Canal (RIC) hearing aids.
In-The-Ear (ITE) hearing aids are designed for arrangement in the ear, normally in the funnel-shaped outer part of the ear canal. In a specific type of ITE hearing aids the hearing aid is placed substantially inside the ear canal. This category is sometimes referred to as Completely-In-Canal (CIC) hearing aids. This type of hearing aid requires an especially compact design in order to allow it to be arranged in the ear canal, while accommodating the components necessary for operation of the hearing aid. Hearing loss of a hearing impaired person is quite often frequency-dependent. This means that the hearing loss of the person varies depending on the frequency. Therefore, when compensating for hearing losses, it can be advantageous to utilize frequency-dependent amplification. Hearing aids therefore often provide to split an input sound signal received by an input transducer of the hearing aid, into various frequency intervals, also called frequency bands, which are independently processed. In this way it is possible to adjust the input sound signal of each frequency band individually to account for the hearing loss in respective frequency bands. The frequency dependent adjustment is normally done by implementing a band split filter and compressors for each of the frequency bands, so-called band split compressors, which may be summarized to a multi-band compressor. In this way it is possible to adjust the gain individually in each frequency band depending on the hearing loss as well as the input level of the input sound signal in a specific frequency range. For example, a band split compressor may provide a higher gain for a soft sound than for a loud sound in its frequency band.
It is well known within the art of hearing aid systems to apply an adaptive filter for a multitude of different purposes such as noise suppression and acoustic feedback cancellation.
EP-B1-2454891 discloses a hearing aid system comprising an adaptive filter that is set up to receive as input signal a signal from a first hearing aid system microphone and provide as output signal a linear combination of previous samples of the input signal, wherein said output signal is set up to resemble a signal from a second hearing aid system microphone as much as possible, whereby wind noise induced in the microphones may be suppressed. Thus if:                the signal from the first hearing aid system microphone is denoted x(n) and a first set of signal samples consequently may be denoted xn=[xn, xn−1, xn−2, . . . , xn−N−1]T wherein n is a time index,        the adaptive filter has N coefficients that are denoted w=[w1, w2, . . . , wN]T,        the signal from the second hearing aid system microphone is denoted d(n), then the adaptive filter is set up to operate in accordance with the formula:dn=wnTxn+ε,wherein ε represents noise comprised in the two microphone signals.        
WO-A1-2014198332 discloses a hearing aid system comprising an adaptive filter that is set up to receive as input signal a signal from a first microphone of a first hearing aid of the hearing aid system and provide as output signal a linear combination of previous samples of the input signal, wherein said output signal is set up to resemble a signal from a second microphone of a second hearing aid of the hearing aid system as much as possible, wherein the difference between the output signal and the signal from the second microphone is used to estimate the noise level and wherein the noise level estimate is used as input for subsequent algorithms to be applied in order to suppress noise in the microphone signals. Thus if:                the signal from the first microphone is denoted x(n) and the signal from the second microphone is denoted d(n), then the adaptive filter is also in this case set up to operate in accordance with the formula:dn=wnTxn+ε,wherein ε represents the estimation error that may be used to estimate the noise and wherein the noise estimate is used for improving the subsequent noise suppression in the hearing aid system. In the following ε may also be construed to represent noise generally whereby the term noise is given a relatively broad interpretation in so far that it includes the adaptive filter estimation error.        
There is therefore a need in the art to improve the performance of adaptive filters. In one aspect performance may be increased by minimizing the occurrence of so called artefacts introduced by the adaptive filtering. The occurrence of artefacts may especially be a problem when an adaptive filter has to react fast to sudden changes in the input signal or the desired signal.
It is therefore a feature of the present invention to provide a method of operating a hearing aid system that minimizes the occurrence of artefacts.
It is another feature of the present invention to provide a hearing aid system adapted to provide a method of operating a hearing aid system that minimizes the occurrence of artefacts.